Neutron star may sport four magnetic poles

The neutron star inside the Crab Nebula may have four magnetic poles, rather than the usual two - unlike any other astronomical object known. The poles may have somehow been frozen into the neutron star when it was formed in a supernova explosion.

A neutron star is the dense stellar corpse left behind after a relatively massive star dies in a supernova explosion. Some neutron stars, like the one in the Crab Nebula supernova remnant, are called pulsars because astronomers detect regular radio pulses coming from them.

The pulses are thought to result from lighthouse-like beams of radio energy shooting from the neutron star's magnetic poles that sweep across the Earth as the star rotates.

Usually the beam from only one pole is seen. But sometimes a second, weaker signal can be detected if the beam from the other pole points roughly in Earth's direction when it comes into view. The Crab pulsar has long been known to display weaker, secondary pulses.

Now, observations of unprecedented detail have revealed that the primary and secondary pulses are radically different, casting doubt on the idea that they simply come from opposite magnetic poles. Instead, some astronomers speculate that the secondary pulses are related to an additional pair of magnetic poles.

Different properties

Tim Hankins of the New Mexico Institute of Mining and Technology in Socorro, US, led a team that observed the Crab pulsar with the Arecibo Observatory in Puerto Rico. Their analysis focused on primary and secondary pulses that were especially powerful and could be seen in great detail.

Among other differences, the primary pulses showed a fairly smooth spectrum, while the energy of the secondary pulses was concentrated into a few, very distinct, radio frequency bands.

And the pulses had different durations. The primary pulses were composed of a series of shorter pulses, each lasting less than a nanosecond, while the secondary pulses consisted of a relatively steady signal lasting several microseconds long.

That was surprising, since aside from their polarity, the properties of a pair of magnetic poles should be exactly the same, leading to identical radio emissions, says Hankins. But if the pole producing the secondary pulses belongs to a different pair of poles altogether, the differences would be easier to explain, he says.

"What we think is going on is that there is another pole," Hankins says. "We think we've got a much more complicated magnetic field than the simple magnetic dipole model."

There would presumably be a partner for the third pole, since magnetic poles always come in pairs, he adds.

'Mind boggling'

Neutron stars' magnetic fields are thought to be frozen into them when they formed in supernovae. That suggests the Crab's neutron star may have been given four poles because it collapsed in some complex, asymmetrical way, he says.

David Thompson of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, who is not part of the team, says there may be other explanations for the observations, however. He says some theoretical models have previously attempted to account for the primary and secondary pulses as coming from the leading and trailing edges of a cone of emission from just a single magnetic pole.

But he says the new observations reveal such a disparity between the two pulse types that they are more in line with emission from two separate poles. Even so, he says it is too early to conclude whether these two poles belong to two different north-south pairs or are part of the same pair.

"It's mind boggling the kind of stuff they've been seeing," he told New Scientist. "They don't understand the results, and nobody else does either."

Tiniest object

The observations have revealed other surprises, as well. Because the primary pulse emission lasts just 0.4 nanoseconds, it may arise in an extremely small region just 12 centimetres across, where a cloud of plasma is trapped above the surface of the neutron star.

If this conclusion is correct, the grapefruit-sized patch would be the tiniest individual object ever observed in astronomy, Hankins says.

The results were presented on Monday at a meeting of the American Astronomical Society in Seattle, Washington, US.

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The 6-light-year-wide Crab Nebula was created by a star that blew up nearly 1000 years ago, leaving behind an unusual neutron star that may boast four magnetic poles (Image: NASA/ESA/J Hester/A Loll/ASU)